This invention relates to an image generator for a video display, and in particular, to an image generator for generating accurate graphical images for a video monitor.
There are several medical diagnostic and research instruments that require sinusoidal gratings on an illuminated screen. For example, a method for testing for glaucoma known as frequency doubled perimetry involves having a patient observe a sinusoidal grating pattern in a number of different regions of a display screen of a video monitor. The contrast of the pattern is varied until the patient observing the well known phenomenon of visual frequency doubling notes the cessation of the phenomenon, thereby establishing a contrast sensitivity for that patient. The contrast sensitivity is compared to the contrast sensitivity for persons of normal vision. A significant deviation from the norm in contrast sensitivity may indicate some degradation of the visual function, possibly due to the presence of glaucoma, AMD (age related macular degeneration), diabetic retinopathy, or other disease. Some testing methods are presented in detail in U.S. Pat. Nos. 5,295,495 and 5,065,767 incorporated herein by reference.
Producing the required contrast bars, or grating, requires providing video timing signals and a high resolution gray scale video signal to the video monitor. A conventional approach to generating a video signal of the necessary high resolution includes using a video coprocessor, a 12 bit deep frame buffer, and a very fast bit digital to analog converter (DAC). There are several disadvantages to this conventional approach.
First, nearly 420 KBytes of fast VRAM are required for the frame buffer. Updating such a frame buffer with a new screen pattern in 10 msec., necessary for each contrast adjustment to appear instantaneous to the viewer, requires a data bus bandwidth of 2.6 Mwords/sec (1 word equals 12 bits) along with a DAC that is fast enough to handle the data bus speed.
Second, the expense of currently available high integration video controller IC's capable of such graphics, a very fast DAC, and a 12 bit deep frame buffer add significantly to the overall cost of a conventional video generator adapted to this special use.